(1) Field of the Invention
The present invention relates to an electrical insulating resin composition which may suitably be used as an electrical insulating material in a circuit board such as a printed wiring assembly, as well as an uncured sheet product, an uncured laminate for the circuit board and a cured laminate for the circuit board by the use thereof, and in particular relates to an electrical insulating resin composition capable of forming an electrical insulating layer having excellent flexibility together with high thermal resistance. More particularly, the invention relates to an electrical insulating resin composition which may suitably be used for a metal based board having high thermal radiation property and can form an electrical insulating layer which is excellent in solder temperature resistance and stably usable for a long time even under a harsh environment such as a high temperature and a high humidity.
(2) Description of the Related Art
Recently, electric home appliances, industrial instruments, office automation equipments, and in-vehicle electronic devices are required to be downsized for saving spaces, and to have some additional values. Therefore, insulating materials durable under ever-harsher temperature, humidity and electric environments have been demanded. In particular, electrical insulating materials which are excellent in electrical insulating property, dielectric strength, thermal radiation property, and connection reliability at soldered portions have been desired.
As a circuit board used under such a harsh environment, a metal based board having a metal substrate such as aluminum and copper that are well capable of radiating heat, an electrical insulating layer provided thereon, and a current-carrying circuit arranged thereon has been frequently used.
In such a metal based board, an epoxy resin is often utilized as the electrical insulating layer. However, a thermal conductivity of the epoxy resin itself is low, and thus it can not efficiently transmit the heat generated in the current-carrying circuit to the metal substrate. Thus, the thermal conductivity of the electrical insulating layer has been improved by eminently filling a thermal conductive inorganic filler in the epoxy resin.
However, when the ratio of the inorganic filler is increased for the purpose of enhancing the thermal conductivity, the flexibility of the electrical insulating layer is reduced to become fragile, and additionally adhesiveness of the electrical insulating layer to the metal substrate is also reduced. Upon performing soldering on the metal based board at high temperature of 200° C. or above, if the ratio of the inorganic filler is high, the electrical insulating layer is cracked due to the high temperature at the soldering, which has caused a decrease of the dielectric strength.
Thus, in order to improve the flexibility and the adhesiveness of the electrical insulating layer, Patent Document 1 (JP-P-H11-8450 A) discloses a metal based board using a resin composition with an elastic modulus of 2×1010 Pa or less at −40° C. As an example of the resin composition having such a low elastic modulus, Patent Document 1 discloses an epoxy resin in which a rubber component is dispersed. As specific examples of the epoxy resin in which the rubber component is dispersed, Patent Document 1 exemplifies a CTBN (carboxyl-terminated butadiene-acrylonitrile rubber) modified epoxy resin and an epoxy resin containing a flexible curing agent of modified polyamine. However, electrical insulating layer obtained by the use of such an epoxy resin with low elastic modulus contains ionic impurities, and thus it is inevitable to reduce electric and physical properties such as ion migration resistance and water-absorbing property in a produced circuit. A handling performance has been improved by the use of a semi-cured resin (B stage state) so that the insulating adhesive layer material of such an epoxy-curable resin is not sticky when touched with a hand at room temperature. However, a material lifetime of the insulating adhesive layer material of such an epoxy-curable resin is short in the B stage state, and management and storage thereof have been highly problematic.
Patent Document 2 (JP-P-H10-242606 A) discloses a thermal conductive adhesive composition with low elastic modulus in which acryl rubber or NBR having reactive groups such as an epoxy group and having a glass transition point of 0° C. or less and a molecular weight of 0.1 to 2 millions is added to the epoxy resin, as an insulating adhesive layer material excellent in thermal conductivity, humidity resistance, thermal resistance, insulating reliability and anti-cracking. In the same manner as the material disclosed in the Patent Document 1, handling performance has been improved by the use of a semi-cured resin (B stage state) so that the insulating adhesive layer material of such an epoxy-curable resin is not sticky when touched with a hand at room temperature. However, a material lifetime of the insulating adhesive layer material of such an epoxy-curable resin is short in the B stage state, and management and storage thereof have been highly problematic. If the resin composition described in Patent Document 2 contains the inorganic filler at a high ratio, the flexibility of the electrical insulating layer is reduced to become fragile. Thus there has been a drawback that the inorganic filler can not be eminently filled.
Patent Document 3 (JP-P-H8-208856 A) discloses a thermally curable composition containing a polybutadiene resin or a polyisoprene resin and unsaturated polybutadiene- or a polyisoprene-containing polymer as an electrical insulating layer material utilizing a radical polymerization reaction. However, in Patent Document 3, 1,2-adducts are primarily used as the polybutadiene resin or the polyisoprene resin, and unsaturated butadiene or isoprene. Therefore, a resulting cured product is very hard, and an adhesive strength to a substrate is also low.
Additionally, Patent Document 4 (JP-P-2001-81429 A) has disclosed a method for performing radical polymerization of a composition having epoxy-modified rubber, an allyl group-containing compound and a curable resin component such as bismaleimide. However, it is a major object of Patent Document 4 to provide a material having a low relative dielectric constant in order to accelerate the speed of signal processing, and therefore, the adhesiveness of a resulting electrical insulating layer is extremely low. Due to the low content of the epoxy-modified rubber, the hardness is too high to use in an application which requires flexibility. The material lacks insulation reliability because of flagrant fragility when an inorganic filler is added at a large amount. Eventually, the resulting electrical insulating layer is actually unavailable as an insulating material for thermal radiation.
The following is the list of prior art documents:    Patent Document 1: JP-P-H11-8450 A    Patent Document 2: JP-P-H10-242606 A    Patent Document 3: JP-P-H8-208856 A    Patent Document 4: JP-P-2001-81429 A